The present invention claims priority from Japanese Patent Applications No. 9-050693 filed Mar. 5, 1997 and No. 9-060980 filed Mar. 14, 1997, which are incorporated herein by reference.
1. Field of the Invention
This invention is utilized in radio communications using time division multiple access (TDMA), and is suited for use in mobile communications and wireless LANs which make in-building use of portable radio terminals.
2. Description of Related Art
Demand for high-speed wireless access communications has recently been increasing, and TDMA is one of the principal access systems in this field. Sector antennas are utilized for high-speed wireless access communications in order to suppress multipath interference and obtain efficient frequency utilization. A sector antenna is an antenna which, by using a plurality of directional antennas, is capable of communicating with stations in all directions. In an access system which uses sector antennas, a radio base station successively switches among its sector antennas, thereby transmitting an access channel in all directions covered by the plurality of directional antennas. If TDMA is employed, the sector is switched over at every TDMA frame.
A problem here is that with a high-speed wireless access system, the optimum value for the number of sectors or the number of branches of a radio base station sector antenna differs according to the base station installation environment or radio wave propagation environment. However, if the number of sectors or branches is different at each radio base station, then when a radio terminal communicates with any of a plurality of radio base stations it will have to do so while referring to information on the relation between the method by which the radio base station selects the frequency and which antenna sector to use to transmit the access channel, and the number of sectors used for its transmission. However, when a high-speed wireless access system is implemented with a large number of installed radio base stations, it is difficult to ensure that a radio terminal holds separate information for all these base stations. Even if this is assumed to have been possible, it would still be more difficult to update this information each time the number of installed radio base stations is changed or their specifications are altered. There is therefore a need for a method capable of synchronizing flexibly against changes in the number of sectors used at a radio base station for transmission over the access channel, without holding any information at the radio terminal.
High-speed wireless access systems which involve dividing zones into pico-cells and installing a large number of radio base stations have been studied. However, an additional problem in this case is that access channels from a plurality of radio base stations are frequently received at one radio terminal. Namely, there are a plurality of radio carrier frequencies and the same radio carrier frequency is sometimes used by a plurality of radio base stations. Moreover, because a radio base station uses a sector antenna to transmit the access channels, a plurality of access channels which have been transmitted from one radio base station using different sectors will sometimes arrive at a radio terminal. That is, a radio terminal will sometimes receive the access channel from a plurality of base stations, at a plurality of frequencies or timings, or from a plurality of directions. It is then necessary for the radio terminal to switch among the radio carrier frequencies and sectors it receives, and to select from these the ones giving the best quality access channel. Hitherto, however, there has been no method for selecting the optimum access channel from among the plurality of access channels received by switching among the radio carrier frequencies and sectors the radio terminal receives, and for establishing synchronization on this basis.